1. Field Of The Invention
This invention relates generally to the field of magnetic resonance imaging systems and more specifically to a quadrature antenna coil assembly in a magnetic resonance imaging system which assembly contains a coupling cancellation network to cancel undesired coupling between the two channels of a quadrature antenna assembly.
2. Description Of Related Art
Magnetic resonance imaging ("MRI"), also known as nuclear magnetic resonance ("NMR") imaging, has become a valuable tool as a safe, noninvasive means for obtaining information in the form of images of objects under examination. For example, MRI can be used as a medical diagnostic tool by providing images of selected portions of the human body without the use of X-ray photography. In such an MRI system, a quadrature antenna coil assembly can be used. One such antenna assembly is shown in FIG. 1.
In FIG. 1, MRI antenna coil assembly 10 includes two perpendicularly oriented receiver coils L.sub.x (L.sub.x1, L.sub.x2) and L.sub.y (L.sub.y1, L.sub.y2) to sense the oscillating radio frequency ("RF") magnetic fields generated from the precessing protons P of the object being examined in the antenna coil assembly volume. The RF magnetic fields are sensed along two axes X and Y. These two axes are perpendicular to the Z axis of the system, which axis is in the direction of the static magnetic field used in the MRI system. Receiver coil L.sub.x is tuned with a parallel capacitor C.sub.x comprised of capacitors C.sub.x1 and C.sub.x2 where L.sub.x, C.sub.x forms a resonant circuit. Similarly, receiver coil L.sub.y is tuned by parallel capacitor C.sub.y comprising capacitors C.sub.y1 and C.sub.y2 where L.sub.y, C.sub.y also forms a resonant circuit. Each receiver coil is tuned to resonate at the appropriate signal frequency. In the typical quadrature antenna coil assembly shown in FIG. 1, the signals received in the X axis and in the Y axis are preamplified through X amplifier 11 and Y amplifier 12, respectively, and fed into quadrature combiner 13 as the X and Y channels of the system. Quadrature combiner 13 provides output signal S.sub.o which is processed by the remainder of the MRI system (not illustrated) to provide an intelligible image of the selected portion of the object being examined.
Each channel's receiver coil is similar to a typical linearly-polarized coil commonly used for MRI receiving. In the ideal situation, each receiver coil should show a high degree of linear spatial polarization, with a large ratio between the maximum and minimum response to a linearly-polarized field. Also, the maxima and minima on one receiver coil should be rotated by 90.degree. with respect to the maxima and minima on the other receiver coil. Combining the outputs from the X and Y channels in a quadrature combiner would, in such a situation, show an ideal response to a circularly-polarized field, such as from the precessing spins of the protons P of the body being examined by the MRI system. In such an ideal situation, there would be no inductive coupling between the two receiver coils. In practice, however, the use of an MRI antenna assembly, such as assembly 10, suffers from undesigned and undesired coupling between the X and Y channels. These coupling mechanisms include small mutual inductances between the coils, capacitance from one coil to the other, and coupling through other circuits, such as other coils, in the MRI system.
Since the resonant circuits L.sub.x, C.sub.x and L.sub.y, C.sub.y of MRI antenna assembly 10 are high Q circuits, the coupling between the two receiver coils in the quadrature system is greatly magnified. For proper imaging, the requirements of the antenna system are very strict, and a small coupling coefficient factor k will, therefore, lead to poor quadrature imaging results.
As an example of the high Q coupling magnification problem, a three-turn receiver coil set may have a coupling factor of the absolute value .vertline.k.vertline. between the two receiver coils of less than 1%. However, since the Q is greater than 200 for the circuit, the magnified coupling .vertline.k.multidot.Q.vertline. is greater than 1. For proper quadrature receiving, it has been determined that the magnified coupling .vertline.k.multidot.Q.vertline. must be less than 0.1. Thus, the coupling factor .vertline.k.vertline. must be reduced to less than 5.times.10.sup.-4.
Attempts have been made to reduce the unwanted coupling between the receiver coils of MRI antenna assemblies. Such attempts include, for example, shielding the coils from each other. Those attempts, however, have had difficulty in reducing the coupling sufficiently to provide clear and, thus, more useful quadrature imaging. Additionally, such shielding may require additional manufacturing steps, can be difficult to install, and is an unwanted, additional expense.
From the foregoing considerations, it should be apparent that there is a great need for an improved MRI antenna assembly in which the problems of unwanted coupling between receiver coils is eliminated.
It is, thus, an intention of the invention to provide a network for use in an MRI antenna assembly to eliminate the undesired coupling between the receiver coils.
Another intention of the invention is to eliminate the undesired coupling in an MRI antenna assembly by a simple, easy to adjust, reliable, stable, and inexpensive circuit.
Still another intention of the invention is to provide a coupling cancellation network to an MRI antenna assembly in which the undesired coupling between the two receiver coils is cancelled by the creation of additional coupling between the two receiver coils which is the opposite polarity of the undesired coupling and in equal magnitude to the undesired coupling by providing a neutralization coupling factor k' to cancel the assembly's normal coupling factor k.
Yet another intention of the invention is to allow for the additional coupling to be easily adjustable.
A further intention of the invention is to provide circuitry for the additional coupling network to reverse the polarity of the additional coupling as needed.
Other intentions and features of the invention will further become apparent with reference to the accompanying drawings and detailed description of the invention.